The invention relates to ink-jet ink compositions, and more particularly to a process for the preparation of such ink-jet ink compositions suitable for use in the preparation of MICR documents. Specifically, the invention relates to aqueous ink-jet ink compositions containing extremely small particle size conductive metal oxides that have been treated to enhance performance and suspension parameters, and particular grinding, milling and filtration processing techniques to aid in the enhancement of the performance of such ink-jet ink compositions.
Conventional aqueous ink-jet ink compositions contain a dye or pigment, a solvent system, which may be aqueous or non-aqueous in nature, and which may include a combination of various solvents or a single solvent, and various other components such as humectants, surfactants, dispersion aides, biocide or fungicide, and other components. These components are for the most part known in the industry, and when combined according to conventional techniques for ink-jet ink processing and preparation, create inks well suited to various printers and the specific print processing parameters of these printers. The known techniques include mixing in conventional equipment, such as milling equipment.
Of particular interest in this invention are inks suited for use in drop-on-demand and continuous print processing and which contain a conductive metal oxide component. These inks are generally categorized as magnetic inks, and may be used in various applications. One such application is the preparation of magnetic ink character recognition, or xe2x80x9cMICRxe2x80x9d, readable documentation. This type of application involves the use of magnetic ink to print all or part of a document, usually for security purposes For instance, some xe2x80x9cdocumentsxe2x80x9d where MICR readable ink compositions are used include checks, bonds, security cards, etc. MICR ink may be used to print an entire document, or only a portion thereof. For instance just the bar code region or only certain characters may be printed with MICR readable ink. The document, once printed and subjected to an appropriate source of magnetization, is then passed through or under a MICR reading device, which validates or authenticates the document based on the MICR encoded characters or printed matter. The step of magnetizing the MICR ink once printed and before use imparts a specific magnetic charge to the magnetic component of the ink, causing an alignment of the particles. The particles must then retain the magnetic charge. The capability of a magnetic material to retain the imparted charge is referred to as remanence. Generally, this parameter increases with an increase in the particle size of the magnetic material. However, the larger the particle size the more difficult it is to maintain the particle in suspension within an ink composition. Additionally, the print head nozzles of current ink-jet printers are very small, therefore the particulate matter in an acceptable ink-jet ink must be small in order to avoid clogging the nozzles, whether during printing or over an extended period of time.
This need to maintain high remanence, but to decrease the particle size of the magnetic material to a very small size, presents a unique problem for potential MICR ink-jet ink manufacturers. The challenges of formulating a suitable ink-jet ink for use in MICR printing applications revolve around the need to achieve an ink composition containing very small particle size magnetic material, due to the size of the print head nozzles, and yet maintain the necessary level of remanence within the particulate matter in the ink such that the MICR readable characteristic of the ink is not compromised. Remanence is directly proportional to the size of the particle, thus decreasing the particle size of the magnetic material in order to avoid nozzle clogging of the print heads may also decrease the remanence parameter of the ink. One means of addressing a loss of remanence is to increase the magnetic component loading. This, however, is difficult due to the tendency of the particulate matter to settle out of solution as the amount of particulate matter in the ink composition is increased
Various attempts have been made to address the problem of retaining the magnetic pigment or particulate matter in suspension. For example, U.S. Pat. Nos. 5,026,427, 5,240,626, and 5,656,071 each suggest the use of specific dispersants to maintain the suspension. The 5,656,071 patent discloses an ink composition including a polymeric dispersant to maintain a metal oxide in solution and a co-solvent mixture of 1,3-propanediol or 1,4-butanediol with a second solvent selected from polyethylene glycol-type materials and polyol/polyalkylene oxide condensates. The other patents involve the use of colloidally dispersed magnetite in conjunction with a specified dispersant component. U.S. Pat. No. 5,240,626 discloses an ink including colloidally dispersed magnetite particles coated with a carboxy compound-type anti-agglomeration agent and a dispersing aid. The 5,026,427 patent teaches generally the preparation of a magnetic ink composition containing magnetic particles and specific dispersants to maintain the dispersion. Another means of addressing the suspension issue is set forth in U.S. Pat. No. 4,026,713. This patent discloses the use of a combination of surfactants and glycerol to make stable magnetic inks.
Yet another means of addressing the suspension problem of magnetic inks has been the use of resin components to enhance the oxide suspension. U.S. Pat. Nos. 5,547,804, 5,670,078, and 5,969,003 are examples of this type of response to the problem. In U.S. Pat. No. 5,547,804 a solvent-based dispersion is aided using a co-polymer resin. The 5,670,078 patent discloses the use of an ion exchange-type resin to maintain the dispersion. The 5,969,003 patent teaches the use of a sulfonated polyester resin to achieve and maintain the oxide component in suspension.
While several of the foregoing patents suggest peripherally that the use of the stated techniques to maintain particulate matter in suspension may be applicable to magnetic inks, none really addresses MICR ink and the specific set of problems particular thereto, such as particle size, remanence, and suspension, among other concerns. None satisfactorily addresses the need to increase magnetic particle content or loading, and thus maintain high remanence of the overall ink composition, without jeopardizing the dispersion necessary in order to attain an ink with the desired shelf life and one that will not clog the very small print head nozzles used in ink-jet printing equipment. Finally, no reference addresses the problem of maintaining a good dispersion of metal oxide in the ink-jet ink composition from the aspect of processing of the metal oxide to achieve the necessary small particle size.
The challenges of formulating a suitable ink-jet ink for use in MICR printing applications revolve around the need to achieve an ink composition containing very small particle size magnetic material, due to the size of the print head nozzles, and yet maintain the necessary level of remanence within the particulate matter in the ink such that the MICR readable characteristic of the ink is not compromised. The invention relates to a means of achieving the foregoing challenge.
One aspect of the invention relates to a process for the preparation of an aqueous MICR ink-jet ink composition comprising a metal oxide dispersion and a suitable ink-jet ink formulation, the dispersion comprising metal oxide particles of very small size, less than about 0.5xcexc, wherein the preparation process includes preparing the metal oxide pre-dispersion and then grinding the pre-dispersion to reduce the size of the magnetic oxide particles prior to the addition of the pre-dispersion to the remaining ink-jet ink formulation components.
Yet another aspect of the invention relates to a process for the preparation of a MICR ink-jet ink composition comprising a metal oxide dispersion and a suitable ink-jet ink formulation, the dispersion comprising metal oxide particles of very small size, less than about 0.5xcexc wherein the preparation process includes preparing the metal oxide pre-dispersion and then grinding the pre-dispersion using a combination of conventional and non-conventional grinding techniques to reduce the size of the magnetic oxide particles and to reduce the potential for particle agglomeration.
Still another aspect of the invention relates to a process for the preparation of a MICR ink-jet ink composition comprising a metal oxide dispersion and a suitable ink-jet ink formulation, the dispersion comprising metal oxide particles of very small size, less than about 0.5xcexc, prepared by first obtaining a suitable metal oxide pre-dispersion, and then grinding and filtering the pre-dispersion to reduce the size of the magnetic oxide particles and to reduce the potential for particle agglomeration. The filtering process may involve the use of a single filter or may involve the use of a step-down filtration regimen.
Still other inventive aspects will become apparent to the skilled artisan from a reading of the Detailed Description of the Invention which follows